Usually dark coloration is a good thing. When betta fish become pale, it can often be associate with something wrong with the water (although I also know that when betta fish sleep, their colour tends to fade.)

A good indicator that something's wrong would to look at his gills and see if he seems to be rapidly/heavily/having troubles breathing. If they are, there might be something wrong with the water.

I'd recommend investing in Nutrafin Cycle, it's a product that contains beneficial bacteria that you add straight to your filter and supposedly cycles your tank on the spot. I always let it sit for an hour or two so that all the water is treated before I put the fish in to be safe.